Leakage into the environment of petroleum products, including gasoline, may be damaging to surrounding soil and water. Once a leak is detected, clean up or remediation may be costly and time consuming. Thus, it is desirable to identify leaks as early as possible.
In a dispensing operation, such as a gas station, fuel is typically stored in underground storage tanks (“UST”) from where it is pumped through various conduit lines to an above ground dispensing unit for dispensing into motor vehicles or the like. Leaks of fuel from the tank or from the interconnecting conduit lines to the dispensers can cause significant environmental damage. The United States Environmental Protection Agency (“EPA”), as well as regulatory bodies in many foreign countries, have set certain standards for the detection and prevention of environmental leaks of fuel. For example, as this application is filed, the EPA requires detection methods sufficient to detect volumetric leak rates of 0.1 gallons per hour (gph). It accordingly has been a goal of manufacturers of this equipment to detect leaks and to meet EPA standards in this regard.
A number of devices operating on a variety of physical principles have been proposed to meet these standards and thereby warn of leaks and provide means for stopping the leaks as quickly as possible to reduce the impact on the surrounding environment. By way of example, in fuel dispensing operations, one such type of leak detector device includes a valve disposed in the conduit line having a spring-biased valve element movable toward and away from an associated valve seat. When the pressure drop across the valve element reaches a certain threshold, the valve element moves away from the valve seat against the spring force to allow fluid to flow through the valve and toward a dispensing unit, from where the fuel is dispensed to a motor vehicle or the like. When the dispensing unit is closed or turned off, the pressure drop across the valve element equalizes and the spring force urges the valve element back toward the valve seat and into a closed position so as to prevent any fuel from passing through the valve.
The leak detection function of these devices is typically provided by a bypass line around the valve such that one end thereof is in fluid communication with a first side of the valve (e.g., downstream side) and the other end of the bypass line is in fluid communication with a second side of the valve (e.g., upstream side). A flow detector is typically disposed in the bypass line so as to detect any flow therethrough. Additionally, the bypass line typically has a relatively small cross-sectional area relative to the primary flow path through the valve so as to allow relatively small leaks in the conduit line to be detected.
In operation, when the dispensing unit shuts off (such as after a filling operation), fluid pressure on each side of the valve equalizes and the valve closes. Ideally there is no leak in the conduit line and thus no flow through the bypass line. If, however, there is a leak in the conduit line downstream of the valve, the pressure in the downstream conduit line will steadily decrease. This pressure drop will, in turn, cause fluid to flow from the upstream side of the valve (e.g., high pressure side) to the downstream side of the valve (e.g., low pressure side) through the bypass line. The flow detector will then detect this flow through the bypass line and cause an alarm condition which may shut down the dispensing system to prevent any further leakage of fuel from the conduit line and to the surrounding environment.
Leak detection devices operating on the basic principles outlined above are generally known in the art. By way of example, U.S. Pat. No. 3,940,020 to McCrory et al.; U.S. Pat. No. 3,969,923 to Howell; U.S. Pat. No. 5,014,543 to Franklin et al.; U.S. Pat. Nos. 5,072,621 and 5,315,862 to Hasselmann; and U.S. Pat. No. 5,918,268 to Lukas et al. generally show a valve, a bypass line, and some type of flow detector for detecting flow through the bypass line. These references differ primarily in the flow detector used to detect flow through the bypass line. For example, McCrory et al. and Howell use a reed switch in conjunction with a magnetized piston to sense flow through the bypass line. Franklin et al. use a rotometer to measure the fluid flow through a bypass line. Moreover, Lukas et al. utilize a thermal flow meter that operates on generally well know principles for determining the flow through the bypass line.
While the leak detectors described above generally operate for their intended purpose, there are some drawbacks that make the use of such devices problematic in fuel dispensing operations. For example, such leak detectors generally represent a “choke point” in the overall dispensing system that restricts the delivery of fuel to the dispensing unit. As a result, the delivery of fuel to motor vehicles or the like may appear relatively slow, leading to increased dispense times and increased customer dissatisfaction. Additionally, the effects of such flow restriction may be exacerbated when there are multiple users on a single fluid conduit line.
The flow restriction through these types of devices is believed to be due to the multi-function nature of the return mechanism used in the valve. In many of these prior leak detection devices, for example, a spring is used to urge the valve element toward the closed position. In addition, the spring also retains the valve element in the closed position until the pressure drop across the valve element reaches the threshold level and thereby moves the valve element away from the valve seat. Furthermore, the spring may ensure proper seating of the valve element in the valve seat when the valve element is moved to the closed position. As a result of such a multi-function mechanism, the spring constant of the spring is typically relatively high. The relatively high spring constant not only results in a large pressure drop to initiate movement of the valve element (e.g., cracking pressure) away from the valve seat, but also requires an even larger pressure drop to sustain the valve element in the opened position as the spring elongates (e.g., linear spring). Thus, the spring works against the flow of fluid through the valve and, for a given operating pressure (determined by the submersible pump in the tank), operates to limit the flow therethrough.
Consequently, there is a need for an improved line leak detector that cannot only detect small leaks in a fluid conduit line so as to meet or exceed EPA standards, but also does so while eliminating or minimizing any restriction of flow through the leak detector.